People have seen the symbols since childhood, and as far as I know in every European country it's necessary to pass the exam on knowing what they mean before starting practical driving lessons.
The US doesn't have nearly that issue, since we have tons of symbolic signs, and few languages, which share a ton of close cognates anyway. For instance, most Mexicans I've heard will use the verb "parquear" or "aparcar" to mean "to park" rather than "estacionar" - so as you might expect, they have zero trouble decoding a sign saying "NO PARKING 9AM - 6PM" which is one of the most complicated text-only signs you usually see.
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